fix(frontend): dedupe identical banners in banner.ts

Multiple independent requests that fail with the same message (e.g.
Activities.tsx's mount effect firing loadFirstPage/listWorkoutKinds/getProfile
separately) each call showError() independently, previously stacking
2-3 textually-identical banners for a single root cause (backend down,
brief network blip). show() now skips adding a banner if one with the
same severity+message is already visible, and skips scheduling a
redundant auto-dismiss timer for it -- the original banner's timer
keeps running untouched.

frontend/src/banner.ts

@@ -17,6 +17,11 @@ function notify() {
 }
 
 function show(severity: BannerSeverity, message: string): void {
+  const isDuplicate = banners.some(
+    (b) => b.severity === severity && b.message === message,
+  );
+  if (isDuplicate) return;
+
   const id = nextId++;
   banners = [{ id, severity, message }, ...banners];
   notify();